Location of conductive faults in telephone and similar shielded cables is most often accomplished by measuring the resistance of one of the faulted conductors between a reference point and the fault. Actual distance is determined by calculation using the known resistance per unit length and the measured resistance.
Current practice is to make the resistance measurement with an electrical bridge which is made up of the faulted conductors and one or more non-faulted conductors which may or may not be in the same cable. The arrangement is such that the fault resistance ends up in one of the power supply leads to the bridge and not within the bridge loop itself. The conductor between the reference point and the fault; however, becomes one of the bridge arms and its resistance is determinable by ratiometric means to the resistance of the non-faulted conductor. The resistance of the non-faulted conductor is known or is readily measureable, since its end point is known and accessible. Conventional forms of the bridge arrangement are known as Varley bridges and Murray loops and are well covered in contemporary engineering literature.
These bridges have shortcomings which make them difficult to use in some situations, notably when any of the parameters or external interfering voltages are varying. Often the fault is due to water in the cable. This causes conduction between conductors or from a conductor to the shield which, when it becomes great enough, constitutes a fault. Other conductors, which have voltage on them may have faults to the faulted conductors and thus inject interfering currents into the fault. Electrical current through any of the water based paths will both electrolyze the water and tend to dry out the fault. Furthermore, the non-faulted reference conductor and its connection to the faulted conductor is a temporary arrangement which may have unexpectedly large and unstable series resistance. In this condition bridge parameters will be varying with time and it is more difficult to balance the bridge. Furthermore, the mere task of balancing a bridge, even when the parameters are constant, is a slow and tedious process.
It is the objective of this invention to provide an improved method for locating conductive faults in telephone and similar shielded cables.